Semiconductor switches, such as Insulated Gate Bipolar Transistors (IGBT), Metal Oxide Semiconductor Field Effect Transistors (MOSFET), and diodes are widely used in various power electronic systems. These devices have precise ratings and are prone to failure under abnormal conditions, such as over-current/voltage, excessive temperature, bad gate drive, and turn on/off at the wrong instances. Also, the devices deteriorate over time due to wear out mechanisms and eventually fail, especially under heavy load and wide temperature/power excursions.
Under generally normal conditions, the turn-on process of semiconductor switches includes three stages, as illustrated in FIG. 1. First, there is a time delay t.sub.d from when the control signal (gate signal) is applied to when the device starts to respond. After t.sub.d, the device's switch voltage/current goes through a fast transition period, and this interval is normally named rise/fall time (t.sub.f). Finally, the on-state voltage of the device may exhibit a long voltage tail before reaching a steady-state final value. This turn-on process and the final on-state voltage are very important indicators as to how well the circuit operates and how healthy the semiconductor device is.
The switching wave forms and the on-state voltage can be badly distorted under fault conditions. The first potential fault condition is shoot through. It occurs when one or more semiconductor switches are turned-on when as a stiff voltage source is imposed across the devices. Under this condition, the current passing through the devices can be over ten times the rated current of the devices, and the voltage across them is very high. Under this fault condition, the devices can be destroyed in tens of microseconds unless proper action is taken. FIG. 2 illustrates a typical wave form, showing the gate signal, switch voltage and current, through a device during a shoot-through condition. One will note that both the switch voltage and the switch current are very high. To address this concern, generally current or preferably voltage sensors are employed to detect such a catastrophic condition.
One particular system employed to address the shoot-through problem configures a circuit so that a device on-state voltage is compared to a fixed voltage reference. When the on-state voltage exceeds a certain value, a flag is raised to reflect that fault has occurred. However, to avoid nuisance flags caused by device turn-on transient voltage tail under normal process conditions, the voltage reference in this system cannot be set too low. The voltage reference must be set at least a few times the rated on-state voltage drop, which makes this system only useful for catching disastrous or significant faults. It will miss faults that occur at lower levels of voltage.
Another fault condition that can occur is over-current. Due to over-load or short-circuit through low-impedance conditions, the current can surpass the ratings of the devices and impose excessive stresses. The only practical way to detect the over current conditions is by current sensing since even under one hundred percent over-current conditions, the on-state voltage may be less than twice the rated on-state voltage. Voltage detection to determine this type of fault is impractical, and so a difficulty arises in detecting this type of fault with a typical general fault detection arrangement.
Still another type of condition that may occur and eventually lead to a fault is device deterioration. Under heavy loads and excessive temperature/power excursions, solder layers, internal interconnections (e.g. wire bonds) and semiconductor cells gradually deteriorate and eventually cease functioning. All of these phenomenon lead to a reduction in the usable area. Due to this, the current density through the remaining functioning cells gradually increases, which increases power loss, speeds up the deterioration process, and increases the on-state voltage drop. FIG. 3 illustrates a graph of the on-state voltage of a typical IGBT device as a function of time when cycled by rated load. The graph shows how the voltage rises over time, but this condition is still very hard to detect because the voltage change is not significant when compared to a shoot through condition (i.e., only about twenty percent), even when the device may be close to total failure. It is desirable to have an effective on-line device that can detect this device deterioration condition.
Another fault condition that occurs and so needs to be detected is a gate drive failure. Due to the improper design of a circuit or component failure, the gate drive circuit may not provide enough capability to properly drive the semiconductor switches. This can cause excessive switching loss due to slower turn-on/off processes, and elevated conduction loss since the on state voltage drop is higher. While this condition can be detected by monitoring the device turn-on process or gate drive outputs, it adds additional cost and complexity.
Thus, it is desirable to provide a system and method for accurately detecting the various types of fault conditions for these switches, while minimizing the cost and complexity for this detection. Consequently, this invention provides a simple system and method to monitor the on-state voltage to adequately detect all the above mentioned fault conditions.